1. Field of the Invention
The present invention relates to a coating agent (hereinafter, coating agent for UV curable inkjet printing) to be used to form an under coat before the printing and a finishing coat after the printing and curing in UV curable inkjet printing using nonaqueous UV ink.
2. Discussion of the Background
A recording substrate has generally been known that improves characteristics of printing by inkjet inks and that includes a base sheet, such as a normal paper or an inkjet printing paper, and a porous inkjet ink receiving layer that is formed on the base sheet and that is made of fine particles such as amorphous silica and a water soluble binder resin such as polyvinyl alcohol (see JP-A-64-011877).
As for base sheets such as a plastic film or a plastic card, there have been proposed (1) an inkjet recording sheet including a transparent synthetic resin sheet and an ink receiving layer, containing polyvinylpyrrolidone, formed on the surface of the transparent synthetic resin sheet (see JP-A-61-032788), (2) a recording sheet including a transparent thermoplastic resin film and a transparent ink receiving layer, containing aqueous resin such as polyvinyl alcohol or gelatine and colloidal silica, formed on the transparent thermoplastic resin film (see JP-A-61-019389), (3) a recording sheet including a transparent plastic film and a water soluble polymer thin film, containing ultrafine particles having average particle diameters from 1 μm to 100 μm and fine particles having average particle diameters from 1 μm to 20 μm, formed on the transparent plastic film (see JP-A-61-280983), (4) an overhead projector film including a transparent film which is coated with a transparent adhesive containing transparent fine particles (such as glass beads and synthetic resin spherical micro beads) having particle diameters from 5 μm to 50 μm (see JP-A-61-024494), and (5) a method of forming an ink receiving layer by applying heated solution, containing at least two components including a water-soluble polymer compound (e.g., polyvinylpyrrolidone) and a water-insoluble polymer compound (e.g., a condensation product of sorbitol and aromatic aldehyde) and a solvent being a good solvent to one of the two components and a poor solvent to the other component, directly to a base, and drying and removing the solvent (see Japanese Patent No. 2694042).
In the technologies described in the above patent documents, however, aqueous dye or hydrophilic pigment is used as colorant. This means that the resulting recording substrates have insufficient water resistance and poor abrasion resistance. However, a typical condition required for printing on, for example, a plastic card is a highly advanced resistance, as compared to a case where printing is performed on a normal paper.
For this, a printed surface is protected by over coating. For the over coating, an ultraviolet curable ink (paint), an aqueous ink (paint), or a solvent-based ink (paint) may be widely employed. However, the aqueous paint is not practical because it requires time for a drying process and the drying affects the base. Further, the solvent-base paint has problems in that the solvent evaporates into the surrounding air due to drying and thus affects the working atmosphere and worker's health, also leading to a burden on the global environment.
Under the aforementioned circumstances, the following technologies have been proposed.
In a method of printing an image on an object, it has been proposed to use a thermoplastic resin supplied by Valspar of High Point, N.C. as a coating material used in a step of forming an under coat, which has a high surface tension in the range of 38-50 surface dynes and should be applied in a smooth coat without dry spray to maximize ink droplet formation. As a top coat applied to the outer surface after the step for applying the ink droplets, a clear (transparent) varnish has been proposed (see JP-A-2005-521578).
Also, a method for producing an inkjet printed matter has been proposed in which a white undercoat layer (I) is formed on a base by using a white paint containing silica particles of which the oil absorption is 10-400 ml/100 g and having a pigment volume concentration of 5-90%, a printed layer (II) is formed on the white undercoat layer (I) by inkjet method using an aqueous ink composition, and a clear coat layer (III) is formed on the printed layer (II) by using a clear paint (see WO2002-100652).
However, when the technologies described in JP-A-2005-521578 and WO2002-100652 are adapted to a printing system using UV curable inkjet inks, there are limits on improvements in adhesion relative to glass and metal, and in abrasion-resistance of printed matter.